Arnaldo,
The new Corona fuel pump does seem to give larger pressure fluctuations than its predecessors. I think the Corona pump has a smaller internal volume on the pressure side than the older AC & Pierburg pumps. The older pumps also had a space where air got trapped & this may have helped by acting as a sort of damper. A few of my customers with the Corona pump have recently changed over teflon lined hoses which are stiffer than the original rubber hoses and have reported an increase in pressure (probably as a result of the increased pressure fluctuations) fluctuations. A real simple test would be to install a long rubber fuel hose between the pump & the carbs and see if the pressure fluctuations improve.
So based on Conrad's sketch and explanation, I would assume that if the terminus of the drain line was in a high or low pressure location that the pump volume and pressure could be decreased by the incomplete movement of the diaphragm.
Actually, it would explain a lot of pressure fluctuations as the stroke of the diaphragm could vary with the pressure in the drain line.
I just installed one of the new pumps and my pressure is very low under load. If I turn on my Facet pump the pressure is normal. (replaced pressure sending unit / the original problem)
I have the drain line tied to my carb overflow drain line and it extends an inch or two below the cowl.
I will experiment with raising it up into the cowl out of any air flow and see if that helps.
The fuel pump drain or the oil tank vent tube should never be placed in a high or low pressure area. Usually outside the cowl isn't a great spot. Just about 1/2"- 1" inside the cowl should keep it from excessive high or low pressure.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
Ok, so it isn't the drain line in the slip stream causing my problems. I shortened it several inches and tucked it against my fire wall away from any air movement. No change. I still need my aux pump to get my fuel pressure in the green.
I put my old Pierburg pump back on and my pressures are perfect with no fluctuations.
I see in the new installation manual that it calls for a bypass with a check valve around the Aux pump.
I guess the Facet pump my be too restrictive for the new style pumps.
So much for new and improved.
I will try that next.
The positioning of the pump drain needs to go to an atmospheric pressure region. If the drain pipe goes to an area of higher or lower than atmospheric pressure area the fuel pressure will increase or reduce accordingly.
To confirm this i expose what happened to my I.C.P. Savannah vg with 912 uls.
When, due to mandatory Rotax bullettin, i changed old style fuel pump with new corona unit fuel pressure on the ground was stable at all power settings and rpm (around 6 PSI | 0,41 bar).
But in flight at wot fuel pressure drop at the lower limit [/b](around 2 PSI | 0,13 bar). This was not the usual few seconds pressure drop of corona pumps mentioned in rotax bulletins. Pressure always remains at this very low level.
With electric pump on there was no problem.
So the problem was on corona pump and was related to speed. Easy guess from info on this forum was the drain line goes in higher/lower pressure area during flight. Moving drain line position in the plane (few inches!) solved the issue.
Happy ? . Yes but now there was a question.
When flying pressure variation on the airplane (wings, fusolage etc) are very small. Around a tenth of psi (10-20 mbar). I personally measured these pressure variations in the pump drain pipe zone with a arduino board connected to this cheap and excellent sensor arduino pressure sensor and confirmed these values.
now HOW CAN THIS LITTLE PRESSURE VARIATIONS AFFECT A PUMP DIAPRHAM OPERATING AT 10-30 TIME THIS VALUES (0,10-0,20 psi on drain tube VS 2,2-5,8 psi on pump diaprham) ?
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